1. Field of the Invention
This invention relates to force resistance devices, and more particularly to devices that can be connected to two objects for resisting relative movement between the objects.
2. Description of the Related Art
Motion or vibration damping devices, such as machinery or engine mounts, shock absorbers, struts, etc., are well known. Many of these devices include hydraulic fluids or, more recently, field responsive fluids that exhibit changes in flow characteristics when subject to electrical or magnetic fields, such as electroactive fluids, ferro fluids, magnetic fluids, etc.
Included in the class of electroactive fluids are electrorheological (ER) fluids and electrophoretic fluids. ER fluids typically comprise a non-conducting, electrically insulating liquid or liquid mixture, such as silicone-based oils, mineral oils, hydrocarbon oils, etc., and a dispersal of solid particles that exhibit a measure of conductivity or semiconductivity within the oil. ER fluids exhibit Newtonian flow characteristics in the absence of an electric field, such that their shear rate is directly proportional to shear stress. However, when an electric field is applied, no shearing takes place until the shear stress exceeds a yield value which rises with increased electric field strength. The result can appear as an increase in apparent viscosity of several orders of magnitude. ER fluids are especially attractive due to their low electrical power requirements and their rapid and reversible response characteristics. Electrophoretic fluids are suspensions similar to ER fluids but respond quite differently in the presence of an electric field. Electrophoretic fluids typically include particles in the suspension that exhibit strong polar migration characteristics. Under an applied field, the electrophoretic fluid separates into particle-rich and particle-deficient phases as the particles migrate to a charged electrode. For example, the particles in a "positive" electrophoretic fluid migrate toward a positively charged electrode, whereas the particles in a "negative" electrophoretic fluid migrate toward a negatively charged electrode. This process can produce much higher yield strengths at lower operating voltages than ER fluids.
U.S. Pat. No. 5,477,946 issued to Kawamata et al. discloses a vibration damping device that takes advantage of ER fluid properties. The vibration damping device in this patent includes two flexible membrane members having a first end portion connected to a piston member and a second end portion connected to respective faceplates to form two closed chambers. The piston member includes an elongate passage and openings that connect the chambers to each other through the passage. The chambers and passage are filled with an ER fluid. An electrical field can be applied to the ER fluid in the passage to restrict movement of the fluid from one chamber to the other as the piston member is moved.